Intercommunication system

ABSTRACT

A two-way intercommunication system for selectively interconnecting a central station, such as an apartment or office lobby, with any one of a multiplicity of remote units, such as individual apartments or offices of a multi-unit building.

Apr. 17, 1973 United States atem Ter Veen Auth Prahm...

S 60 67 99 i ll 11 INTERCOMMUNICATION SYSTEM 2,263,157 11/1941 2,536,8201/1951 [75] Inventor. gfiillam B. Ter Veen,1C1nc1nnat1, 3,284,5713,491,353 Assigncc: Scovill Manufacturing Company,

Waterbury, C Primary ExaminerRalph D. Blakeslee Assistant ExaminerThomasL. Kundert [22] Filed 1971 Att0meyWood, Herron & Evans [21] App]. No.:121,034

ABSTRACT A two-way intercommunication system for selectivelyinterconnecting a central station, such as an apartl79/37, l H; ment oroffice lobby, with any one of a multiplicity of 340/286 remote units,such as individual apanments or offices of a multi-unit building.

[51] Int. [58] FieldofSearch...............................

References Cited UNITED STATES PATENTS 3 Claims, 4 Drawing Figures2,567,484 lvens l 79/3 7 REMOTE STA. OR UNIT DR, LOCK RELAY sw. OPTIONCIRCUIT, DIRECTO'RY USE LAMP I I I l SIGNAL GENERATOR AUDIO TONE lCONTROL CRCUIT= REVERSE SWITCH A22 AMP.

SPEAKER RR. LOBBY INTERCOMMUNICATION SYSTEM This invention relates tointercommunication systems, and more particularly to intercommunicationsystems for facilitating two-way communication between a centralstation, such as the lobby of an apartment, office building or the like,and a selected one of a plurality of remote units, such as individualapartments or offices.

The utility of intercommunication systems which permit two-waycommunication between a central station, such as the lobby of anapartment or office building, and a plurality of remote stations, suchas the individual apartments or offices of the building, is now wellestablished and accepted. Such systems permit the tenant of an office orapartment to converse with a prospective visitor prior to permitting thevisitor to enter the apartment or office, as the case may be. Suchpreliminary conversation provides the tenant with an opportunity toscreen or interview the prospective visitor, and if so desired to denyentry. In certain instances, for example in the case of door-to-doorsalesmen, a housewife who normally would be too embarrassed to dismiss asalesman to whom she has opened the door without first subjectingherself to his sale appeal, finds it remarkably less embarrassing andsignificantly more convenient when using an intercommunication system tosummarily dismiss the same salesman without a hearing. This thehousewife does in the privacy and security of her apartment knowing thewill never be nearer than a distant lobby.

From the standpoint of personal security, the preliminary screening ofvisitors afforded by intercommunication systems of the type to whichthis invention is directed permits a housewife to safely deny entry to apotential assailant who, in the absence of such screening, may easilyhave gained entry when the housewife unwittingly opened the door inresponse to what she believes is an innocent knock or ring of thedoorbell.

Finally, intercommunication systems for an apartment or -office buildingare desirable from the standpoint of convenience alone. For example, adairy products delivery-man can conveniently inquire of his customer asto the customers specific need for dairy products on a given day. Thisavoids trecking to the apartment, which may be on the fifteenth oreighteenth floor of a multi-story apartment building, only to learn thatthe tenant/customer desires some unusual dairy product which is notnormally carried by the deliveryman, but is inventoried in his truck.

in designing an intercommunication system, a number of factors must beconsidered if the ultimate cost of the system is to be kept within thereasonable bounds necessary for commercial acceptance. Illustrative ofsuch factors is the complexity of the remote station unit. If the remoteunit is unnecessarily complex, the total cost of the system may beexcessive, it being realized any excess in cost of a remote unit due tounnecessary complexity is multiplied by the number of units, apartments,offices, or the like in the building. For example, if in a 200-unitapartment building the remote unit requires an unnecessary relay, thecost added to the system is not just the cost of one relay, but rather200 relays since there are two hundred apartments in the building. Dueto this multiplication factor, it is therefore essential that thecomplexity of the remote unit be kept low if system cost is to beminimized.

Another important factor cost-wise is the number of individual wireswhich are required to interconnect the central station with the variousremote units. If multiple independent wires are required between thecentral station and each remote unit, the total number of independentwires in the system becomes excessively large, the excessiveness beinginfluenced by the multiplication factor noted above, namely, the numberof apartments or offices in the building. Excessive wiring is costly fortwo reasons, namely, the wire itself is expensive as is the electricians labor required to install it.

Another, and equally important, factor in the commercial acceptance ofan intercommunication system is its convenience and ease of use by thetenant. Such systems should have remote units with a minimum number ofcontrols, which controls are also easily understood and used. It must beremembered that the average user is often a woman who does not have ahigh level of mechanical aptitude or familiarity with technical subjectmatter.

It has been an objective of this invention to provide a two-wayintercommunication system for apartments, office buildings, and the likewhich is relatively simple and inexpensive in design, and which can beconveniently used by mechanically unskilled personnel. This objectivehas been accomplished in accordance with certain principles of thisinvention by providing a system in which all stations, both central andremote, have a two-way speaker and are connectable in parallel to a pairof common audio lines, and a single common control line. In addition tothe two audio lines and the single control line which are common to allstations, a plurality of wires corresponding in number to the number ofremote units connect the central station with individual ones of theremote stations. At the central station an audio amplifier is providedin combination with amplification direction reverser which is responsiveto the control line for reversing, under the control of the remotestations, the connection of the amplifier in the audio lines to therebyfacilitate, on an alternate basis, communications to and from the remotestations. Also at the central station is a signal generator, such as anaudio frequency tone generator, and a plurality of call switchescorresponding in number to the number of remote units. The call switchesselectively interconnect the output of the tone generator to the remoteunits via the individual remote unit signal lines. At the remotestation, in addition to the two-way speaker, are four'state or conditionswitches. in the first, or standby, condition the switch means connectsthe remote speaker between its respective signal line and the commoncontrol line to facilitate initiation of an audible signal at a calledremote station in response to actuation in the lobby of that stationscall switch In a second condition of the remote station switch means,termed the talk condition, the remote speaker is connected across theaudio lines to facilitate communication from the remote station to thecentral station via the audio lines and the amplifier which is normallyconnected to amplify communications originating at the called station.In the third, or listening, condition of the remote switch means, theremote station speaker is connected across the audio lines andadditionally .a control signal. is

placed on the control and one audio line to actuate the amplificationdirection reverser to reverse the amplifier connections and permitamplification of signals originating at the central station. In thefourth condition of the remote station signal means a second controlsignal is placed on the control and one audio line to actuate a doorunlocking mechanism at the central station, providing the remote stationhas first been called by the central unit. In this remote switchcondition, the remote station speaker is disconnected from the audiolines.

The system of this invention has a number of novel and unobviousfeatures. For example, by virtue of using a control line and audio lineswhich are common to each station, remote and central, the number ofindependent wires required is kept to a minimum regardless of the numberof remote units in the system. In fact, the number of independent wiresfor a building having n remote stations is equal to n+3. Also, since theremote station requires only a two-way speaker and switch means, thematerials cost of the system, which is directly related to the cost of asingle remote unit by the number of units in the building, is kept to aminimum. Finally, since the remote unit switch means in the standby ornormal condition connects the speaker to the signal line, a call can beinitiated from the lobby without any required cooperation orparticipation by the party being called.

These and other advantages and objectives will become more readilyapparent from a detailed description of the invention taken inconjunction with the drawings in which:

FIG. 1 is a schematic circuit in block diagram format of a preferredembodiment of an intercommunication system incorporating the principlesof this invention;

FIGS. 2A-2C combined is a detailed circuit diagram ofthe preferredembodiment of FIG. 1.

With reference to FIG. 1, the preferred embodiment is seen to include aplurality of identical remote stations or units -1, 10-2, 10-n, and acentral station or unit 12 to which each ofthe remote units 10-1, 10-2,10-n can be selectively interconnected for two-way communication. Theremote units 10-1, 10-2, 10-n preferably are located in differentapartments or offices of a multi-apartment or multioffice building,while the central unit 12 is located in the lobby or other common areaof the building. In the preferred embodiment the central unit 12includes two identical stations 16 and 16' each of which has a two-wayspeaker 17, 17, a door unlocking mechanism 18, 18', a call panel 19,19', and a use lamp 9, 9'. The speakers l7, 17 of the stations 16, 16'can be selectively interconnected for twoway communication with similartwo-way speakers, to

be described, in the remote units 10-1, 10-2, l0n.,

The door unlocking mechanisms 18, 18 of stations 16, 16 are associatedwith normally locked doorswhich are designed'to be under the control ofdoor switches,

to be described, associated with each remote unit 10-1, 10-2, 10 -n. Thecall panels 19, 19 facilitate selec--=, tive signaling from thestations16, 16' to the various remote units l0-l, l0-2, 10- n for the purpose ofinitiating a communication between one of the central stations 16 or 16and a selected remote unit. For this purpose, each of the call panels19, 19' includes a plurality of. switches20-l, 20-2, 20-n and 20-1',20-2, 20-n', respectively, corresponding in number to the number ofremote units 10-1, 10-2, 10-n, and which when activated, result in theproduction at the corresponding remote unit of an appropriate signalsuch as an audible tone. The use lamp 9 of central station 16 becomesilluminated when central station 16' is in use. Likewise, lamp 9'signals when station 16 is in use.

In accordance with a preferred form, the identical central stations 16and 16 are located in the rear lobby and front lobby, respectively, ofthe building in which the remote units 10-1, 10-2, l0-n are located. Insuch case, the door unlocking mechanisms 18 and 18' control normaliylocked doors between the rear and front lobbies, respectively, and theinterior of the building wherein the remote units 10-1, 10-2, 10-n arelocated.

The central station 12, in addition to the rear and front lobby stations16 and 16', also includes an amplifier 22, a reversal switch 23, a lobbyspeaker selector switch 8, an audio tone signal generator 24, and acontrol circuit 25. The amplifier, reversal switch, selector switch,signal generator and control circuit may be physically located in eitherthe front or rear lobby, or elsewhere in the building. The amplifier 22provides amplification of two-way communications between the rear andfront lobby speakers 17 and 17', respectively, and the speakers of theremote stations 10-1, 10-2, 10-n. The reversal switch 23 controls theamplification direction, that is, whether communications from, or to,the central station are amplified. The selector switch 8 selects theappropriate one of the lobby speakers, front lobby speaker 17' or rearlobby speaker 17, for connection to the amplifier 22 via the reversalswitch 23, depending upon whether the called party is being called fromthe front orrear lobby station 16' or 16, respectively. The signalgenerator 24 generates an audio frequency tone which can be transmittedto one or more of the remote units 10-1, 10-2, l0-n under the control ofthe switches 20-1, 20-2, 20-n and 20-1', 20-2, 20-n' of the respectivecall panels 19 and 19'. The control circuit 25 includes the requisitecircuitry, to be described in detail, for facilitating the signaling,communicating, door-unlocking, and other control functions of thesystem.

The remote units 10-1, 10-2, .10-n, which as indicated are structurallyand operationally identical, each include a two-way speaker 26, atwo-position manually operated door switch 28, a two-position manuallyoperated talk switch 30, and a two-position manually operated listenswitch 32. The remote speaker 26 facilitates two-way communicationbetween its respective remote unit 10-1, 10-2, l0-n and the rear orfront lobby speaker 17, 17'. The door switch 28 facilitates control fromits respective remote unit 10-1, 10-2, l0-n of the rear and front doorunlocking mechanisms 18, 18', facilitating unlocking by personnel at theremote units of the doors in the front and rear lobbieswhich permitaccess to the interior of the building in which the remote units arelocated. The door operated, either front or rear, by switch 28 dependsupon whether the calling party is in the front or rear lobby= 16 or 16,respectively. The talk and listen switches 30 and 32 permit personnel atthe remote units -1, 10-2, 10-n to control the central station reversalswitch 23 and thereby select the direction of permissible communicationbetween the rear or front lobby speakers 17, 17', depending upon whichis in use, and the remote speakers 26. When talk switch 30 is activated,the central station amplifier 22 amplifies speech from the remote unit10 to the central unit 12, and when listen switch 32 is activated, theamplifier 22 amplifies speech from the central unit 12 to the remoteunit 10. The talk and listen switches 30 and 32 each have a normal, orstandby, position in which they collectively connect the remote speaker26 in a manner such that it will audibly reproduce a tone signalgenerated by the tone signal generator 24 should the remote unit besignalled from one or the other of the lobby call panels 19, 19'.

interconnecting the central station 12 and the remote units 10-1, 10-2,10-n are two audio lines A1 and A2. Audio lines Al and A2 are common toeach remote unit 10-1, 10-2, l0-n and alternatively to each of the lobbyunits 16 and 16. Stated differently, the remote units 10-1, 10-2, 10-nand, alternatively, the rear and front lobby units 16 and 16', areconnected in parallel to the audio lines Al and A2. The amplifier 22 isconnected in audio lines A1 and A2 via the reversal switch 23, and,depending on which one of the talk or listen switches 30 and 32 of aremote unit 10-1, 10-2, 10-n is activated, amplifies speech to or fromthe central station 12, respectively. The selector switch 8, alsoconnected in audio lines A1 and A2, is located between the reversalswitch 23 and the front and rear lobby speakers 17' and 17, andfunctions to connect, under control of circuit 25 as shown by line 7,the amplifier 22 (via reversal switch 23) to either the front or rearlobby speaker depending upon which lobby unit 16 or 16' is in use.

Also interconnecting the remote units 10-1, 10-2, 10-n is a control lineC. The control line C, like the audio lines A1 and A2, is common to eachof the remote units 10-1, 10-2, 10-n. Control line C is also connectedto the control circuit 25. Thus, the remote units 10-1, 10-2, 10-n andthe control circuit 25 connect in parallel to the control line C. Thecontrol line is responsive to the position of the remote unit talk andlisten switches 30, 32 for controlling via control circuit 25 thecondition of the reversal switch 23 and, hence, the direction of speechamplifier 22. Control line C is also responsive to the door switch 28for operating, via control circuit 25, the door locking mechanisms 18and 18.

In addition to the audio lines Al and A2 and the control line C, thereare n signal lines S1, S2, Sn. The signal lines S1, S2, Sn arerespectively unique to the remote units 10-1, 10-2, 10-n, that is, eachremote unit has its own individual signal line. Each signal line 81, S2,Sn is connected to the tone signal generator 24 via its respectivelyassociated front lobby call panel switch -1', 20-2, 20-n and itsrespectively associated rear lobby call panel switch 20-1, 20-2, 20-n bylines S and S, respectively. With the signal lines S1, S2, Sn soconnected, the audible tone generated by generator 24 can be selectivelytransmitted to any one of the remote units 10-1, 10-2, 10-n byactivation of the respectively associated front lobby call panelswitches 20-1', 20-2', 20n', or the respectively associated rear lobbycall panel switch 20-1, 20-2, 20-n.

In operation, when a party in the front or rear lobby, herein termed thecalling party desires to communicate with personnel at one of the remoteunits 10-1, 10-2, 10-n, herein termed the called party", the individualin the lobby activates the switch 20-1, 20-2, 20-n corresponding to thedesired remote unit, if in the rear lobby, or the switch 20-1', 20-2',20-n' if in the front lobby. For example, if the calling party in thefront lobby wishes to communicate with remote unit 10-1, the callingparty depresses switch 20-1 of front lobby call panel 19. Depression ofthis switch interconnects the tone signal generator 24 and signal lineS1 of remote unit 10-1, in turn causing an audio signal to betransmitted to speaker 26 of remote unit 10-1. Since talk and listenswitches 30 and 32 of remote unit 10-1 are in their standby positions,speaker 26 of remote unit 10- 1 is connected between switch line S1 onwhich the audio signal is transmitted and control line C, and the toneis audibly reproduced by speaker 26 of called party unit 10-1. Anaudible signal is not reproduced by speakers 26 of the other remoteunits 10-2, 10-n, notwithstanding that they are connected between thecontrol line C and their signal lines S2, Sn through their talk andlisten switches 30 and 32 which are in standby position, because thelobby call switches 20-2, 20-2', 20-n, 20-n' associated with theseremote units have not been activated by the calling party to connect thesignal generator 24 to their respective signal lines S2, Sn.

Activation of a call switch, e.g., switch 20-1 of the rear lobby, inaddition to causing production of an audible signal at the called remoteunit 10-1, also functions to cause the control circuit 25 to operate theselector switch 8 via line 7 to connect the appropriate lobby speaker,in this case rear speaker 17, to the amplifier 22 via reversal switch23.

The audible signal produced by speaker 26 of called remote unit 10-1signals personnel at this remote unit that a caller is in either thefront or rear lobby station 16, 16. At this point, since the talk andlisten switches 30 and 32 of called remote unit 10-1 are in the standbyposition, the remote speaker 26 of unit 10-1 is not connected to thecommon audio lines Al and A2 and the calling party in the front lobbyunit 16' is unable to hear conversation originating in called remoteunit 10-1. If the called party in remote unit 10-1 desires to speak withthe calling party in front lobby station 16', the called party placestalk switch 30 in the operative, or talk, position. With talk switch 30in the talk position, remote speaker 26 is disconnected from signal lineS1 and control line C instead is connected across the audio lines Al andA2. Additionally, control line C is open-circuited when the talk switch30 is in the talk position. This is sensed by control circuit 25 tocause reversal switch 23 to connect the amplifier 22 such that its inputis connected to the remote speaker 26 via lines A1 and A2 and its outputis connected to the rear or front lobby speakers 17, 17, as the case maybe, depending upon where the calling party is located.

When the called party in remote unit 10-1 desires to stop talking, andstart listening in anticipation of hearing conversation originated bythe calling party in the lobby, the talk switch 30 is de-activated andreturned to its standby position, and the listen switch 32 placed in thelisten position. With the listen switch 32 in this position, the remotespeaker 26 is connected to the audio lines A1 and A2. Additionally, apredetermined impedance is placed across control line C and audio lineA2 which is sensed by the control circuit 25 to modify via line 2 7 thereversal switch 23 and in turn switch the input of the amplifier 22 tothe central station speakers 17, 17' selected by switch 8 andtheamplifier output to the remote speaker 26. The calling party in thefront or rear lobby, as the case may be, is now free to speak into thespeaker 17 or 17, which speech is then amplified by amplifier 22 andtransmitted to the remote unit -1 via lines A1 and A2 where it istransduced by speaker 26, which because switch 32 has been placed in thelisten position, is connected to audio lines A1 and A2.

When the called party again wishes to talk, listen switch 32 isde-activated and talk switch 30 is placed in the talk position. Withtalk switch 30 in the talk position, the remote speaker 26 is connectedto audio lines Al and A2 and the predetermined impedance associated withthe talk switch position, e.g., an open circuit, is placed acrosscontrol line C and audio line A2 with the result that the controlcircuit 25 causes, via line 27, the reversal switch 23 to reverse theinput and output of the amplifier 22 to facilitate amplification ofspeech from the remote unit and transmission thereof to the centralstation 12.

Should the called party in remote unit 10-1 desire to unlock the doorconnecting the lobby, either from or rear, whereat the calling party islocated, and the interior of the building in which the remote unit 10-1is located, the door unlock switch 28 is switched from a normal orstandby position to the active position. This is effective to place apredetermined impedance across control line C and audio line A2, in turncausing the control circuit 25 to generate a signal on either line 29'or 29 which activates the door unlocking mechanism 18 or 18 of the frontor rear lobby, depending on where the calling party is located, tounlock that door.

When the called party desires to terminate all conversation, the talkand listen switches 30 and 32 are placed in their standby positionswhich remove speaker 26 from audio lines A1 and A2. This prevents thecalling party from either listening to conversation originating at thecalled station, or from speaking to the called station.

The remote units 10-1, 10-2, 10-n are each structurally andoperationally identical, and therefore only remote unit 10-1 isdescribed in detail. As can best be seen in FIG. 2, the unit 10-1includes the twoway speaker 26 which, when spoken into, transduces thespeech to an electrical signal on its output lines 26-] and 26-2 fortransmission via audio lines Al and A2 to the central unit 12. Thetwo-way speaker 26, when input with a speech-carrying electrical signalon lines 26-1 and 26-2 from the central unit 12 via audio lines Al andA2, transduces the speech signal to an audible output. Two-way speakersof the foregoing type are of conventional and well-known construction,and hence are not described in detail herein. Suitable two-way speakersare commercially available from Nu- Tone Division, Scovill Mfg. Co.,designated Model 36548.

The two-way speaker 26 of unit 10-1 is alternately connectable betweensignal line S1 and control line C, and between audio lines A1 and A2 viathe talk and listen switches 30 and 32. The switch 30 has a first set ofstationary, contacts 51-53 and a second set of stationary contacts54-56. Cooperating with stationary contacts 51-53 is a translatableconductive segment 57, while associated with stationary contacts 54-56is a translatable conductive segment 58. Segments 57 and 58 are ganged,and normally biased by a spring (not shown) to the standby positionshown. Contacts 57 and 58 are movable to an active, or talk, positionshown in phantom lines.

Listen switch 32 has three sets of stationary contacts 59-61, 62-64, and65-67 which cooperate with translatable conductive segments 68, 69 and70, respectively. Segments 68, 69 and 70 are ganged, and normally biasedby a spring (not shown) to the standby position shown. Contact segments68, 69 and 70 are movable to an active, or listen, position shown inphantom lines.

With talk and listen switches 30 and 32 in the standby positions shown,contacts 51 and 52, 54 and 55, 59 and 60, and 62 and 63 are bridged bysegments 57, 58, 68 and 69, respectively. With these contacts bridged,lines 26-1 and 26-2 of two-way speaker 26 are connected across signalline S1 and control'line C. With speaker 26 connected across signal lineS1 and control line C, a tone from the tone signal generator 24 can betransmitted to the speaker 26 of remote unit 10-1 if either the rearlobby panel switch 20-1 or the front lobby panel switch 20-1' isactivated by a calling party seeking to initiate a communication withremote unit 10-1. Since remote units 10-2, 10-n are connected inparallel with remote unit 10-1 to control line C and individually totheir respective signal lines S2 Sn, the speakers 26 of remote unit10-2, 10-n are, when their respective switches 30 and 32 are in thestandby position, connected to the tone signal generator 24 via line Cand their respective signal lines S2, Sn such that a tone can betransmitted to the remote unit 10-2, l0-n upon activation of the frontor rear lobby switches 20-2, 20-2', 20-n, 20-n, respectively.

When talk and listen switches 30 and 32 are in their standby position,audio lines A1 and A2 are open-circuited at the remote unit.

The talk switch 30, in addition to the standby position described above,also has a talk position. In the talk position, segments 57 and 58 aretranslated to the phantom line position such that contacts 52 and 53 arebridged by conductive segment 57 while contacts 55 and 56 are bridged byconductive segment 58. With these contacts bridged, speaker lines 26-]and 26-2 of speaker 26 in remote unit 10-] are connected to audio linesAl and A2, respectively. Lines A1 and A2 are in turn connected to theinput of amplifier 22 via untransferred contacts CRI-A and CRl-B of acontrol relay CR1 to be described, relay contacts CRl-A and CRl-Bforming part of the reverser switch 23. The output of the amplifier 22is connected to the selector switch 8 via untransferred relay contactsCRl-C and CRl-D, respectively, of relay CR1, contacts CRl-C and CRl-Dalso forming part of the reverser switch 23. The selector switch 8, inturn, alternately connects the output of the amplifier 22 to either thefront or rear lobby speaker 17' or 17, depending upon whether thecalling party is in the front or rear lobby. With talk switch 30 of unit10-1 in the talk position, lines S1 and C are open-circuited.

Thus, with the talk switch 30 of remote unit 10-1 in the talk position,speaker 26 of remote unit 10-1 is connected via audio lines Al and A2and the reverser switch 23 to the input of the amplifier 22; whileeither the front or rear lobby speaker 17', 17 is connected to theoutput of the amplifier 22 via switch reverser 23 and selector switch 8,thereby establishing an audio path from the speaker of remote unit 10-1through the amplifier 22 to the rear or front lobby speakers 17 and 17depending on the location of the calling party. The speakers 26 ofremote unit 10-2, l-n, as long as their respective talk switches 30 arein the standby position, are not connected to the input of amplifier 22via audio lines A1 and A2 and the reverser switch 23. Accordingly, onlyspeech input to speaker 26 of remote unit -1, whose talk switch 30 is inthe talk position, will be transmitted to the input of the amplifier andfollowing amplification therein to the front or rear lobby speakers 17'and 17.

The listen switch 32 of remote unit 10-1, in addition to its standbyposition, also has a listen position. In the listen position, conductivesegments 68, 69 and 70 are translated to the phantom line position. Withthese conductive segments so positioned, stationary contacts 60 and 61are bridged by segment 68; contacts 63 and 64 are bridged by conductivesegment 69; and contacts 66 and 67 are bridged by conductive segment 70.Contact 59 which connects to signal line S1 is not bridged to anothercontact. With contact 59 not bridges, line 81 is open-circuited. Thebridging of contacts 60, 61 by segment 68, and bridging of contacts63'and 64 by segment 69 connects speaker lines 26-1 and 26-2 of speaker26 of remote unit 10-1 to audio lines Al and A2, respectively. Thebridging of contacts 66 and 67 by segment 70 connects a predeterminedimpedance R48 between control line C and audio line A2. The presence ofimpedance R48 across lines C and A2 is effective to energize controlcircuit relay CR-l, in a manner to be described, transferring relaycontacts CRl-A to CRl-D of reverser switch 23 from the position shown..With the relay contacts CRl-A to CRl-D transferred, the speaker 26 ofremote unit 10-1, which is connected to lines A1 and A2 via switch 30when in its listen position, is connected to the output of amplifier 22while either the front or rear lobby speakers l7, 17 is connected to theinput of amplifier 22 via selector switch 8. Thus, the placement ofimpedance R48 across control line C and audio line A2, when the switch32 of remote unit 141-] is in the listen position, is effective toenergize relay CR1 and transfer the contacts CRl-A to CRl-D of reverserswitch 23, thereby effectively connecting the appropriate lobby speaker17, 17' to the input of amplifier 22 via selector switch 8 and speaker26 of remote unit 10-1 to the output of amplifier 22.

Switches 30 and 32 of remote units 10-2, 10-n function in a mannersimilar to that of switches 30 and 32 of unit 10-1.

The door control switch 28 of remote unit 10-1 includes a conductivecontact 28-1 movable between an inactive or standby position (shown inFIG. 2) and an active position shown in phantom lines in which itbridges stationary contacts 28-2 and 28-3. in the active position ofdoor control switch 28, a predetermined impedance R49 is connectedacross control line C and audio line A2. This impedance R49 across linesC and A2 is sensed by the control circuit 25 and, in a manner to bedescribed, operates the front or rear door unlocking mechanism 18 or 18,depending upon whether the calling party is at the front or rear lobby16 or 16, respectively.

Door switches 28 of remote units 10-2, 10-n, like door switch 28 ofremote unit 10-1, unlocks the appropriate front or rear lobby door whenplaced in the active, or door unlock position, shown in phantom lines.

The audio tone signal generator 24 and control circuit 25 are describedpresently in conjunction with a detailed description of the operation ofthe system of this invention in its various modes. When a call switch20-1 20-n in the front lobby 16 or 20-1 20-n in the rear lobby 16, forexample, switch 20-1 of the rear lobby 16, is activated, a circuit iscompleted from the positive bus which is preferably at +24 volts to theground bus via resistor R28, resistor R46, control line C, speaker 26 ofcalled remote unit 10-1, signal line S1, activated call switch 20-1,signal line S, resistor R38, and resistor R31. Completion of thiscircuit causes current to flow through resistor R31 raising thepotential of the base of a transistor TR16, rendering this normallynonconducting transistor in a conductive state. Conduction of transistorTR16 causes a flow of current in its emitter-collector path whichincludes resistors R11 and R12 connected between the collector oftransistor TR16 and the positive bus.

Current flow through resistor R11 applies a signal to the base of atransistor TR9 rendering this transistor which is normally nonconductivein a conductive state. Conduction of transistor TR9 causes current toflow fiow through its emitter-collector path to a tone signal generatorcircuit 24 which includes a unijunction transistor TR2, a capacitor C 1,and charging resistors R39 and R15. The flow of current throughconducting transistor TR9 causes the tone generator 24 to produce anaudio frequency signal at its output line 100. The audio frequencysignal present on line is resistively coupled to a tone signal generatoramplifier including transistors TRl and TR17, providing on line 102 anamplified audio frequency tone signal.

The conduction of transistor TR16 in response to closure of call switch20-1 is also effective to switch normally nonconducting transistor TR8to a conductive state. The conduction of transistor TR8 short-circuitsresistor R28 which was in series with the speaker 26 of remote unit 10-1via lines C and S1, permitting a maximum level of audio frequency tonesignal to flow through the speaker 26 of remote unit 10-1 when theamplified output of the tone generator 24 present on line 102 is coupledto common signal line S in a manner to be described.

Completion of the circuit from the positive bus through resistors R28,R46, line C, speaker 26 of remote unit 10-1, line S1, line S, andresistors R38 and R31, to ground bus as a consequence of closing callswitch 20-1 is also effective to bias via diode D23 the base oftransistor TR12, rendering this normally nonconductive transistor in aconductive state. Transistor TR12 is in series with a control relay CR2associated with the rear lobby l6, and when turned on renders controlrelay CR2 energized via a path which includes a contact CR3-A of acontrol relay CR3, to be described, which is associated with the frontlobby 16. Contact CR3-A of relay CR3 associated with the front lobby isin the position shown permitting relay CR2 associated with the rearlobby to be energized in response to closure of a rear lobby callswitch, such as call switch 20-1, when a call switch in the front lobbyhas not been previously energized. Contact CR3-A, therefore, is alock-out contact in the sense that it prevents control relay CR2associated with the rear lobby from being energized if control relay CR3associated with the front lobby is energized by virtue of the prioractuation of a front lobby call switch 20-1 20-n'. Energization of relayCR2 which, as indicated, is associated with the rear lobby, transfersits own contact CR2-D from the position shown, connecting the positivebus to the base circuit of a transistor TR13, energizing normallynonconducting transistor TR13.

Transistor TR13 is in parallel with transistor TR12 and provides analternate energization path for ener gized relay CR2. Thus, transferredcontact CR2D in connection with transistor TR13 whose emitter-collectorpath is in series with relay CR2, function as a latching circuit forrelay CR2. The transfer of relay contact CR2-D from the position shownalso performs two other functions. First, it applies potential from thepositive bus via a diode D and a line 105 to an astable multi-vibrator106 which includes transistors TRS and TR6 cross-coupled in a well-knownmanner to produce at its output terminal 107 a square-wave, preferablyof .relatively low frequency such as two cycles per second. The twocycle per second output at astable multi-vibrator terminal 107 switchesa transistor TR7 at the same frequency. Transistor TR7 has itsemitter-collector path connected between the ground bus and the frontand rear lobby use lamps 9 and 9, causing whichever of these lampshaving its positive side connected to the positive bus 24 to be operatedin a flashing mode. In this example, wherein a call switch -1 in therear lobby 16 is activated to energize control relay CR2, in turntransferring contact CR2-D from the position shown, no connection existsbetween the positive bus and the positive side of the rear lobby uselamp 9. The use lamp'9 therefore will not operate in a flashing mode inresponse to the intermittent grounding of the negative side of the lampby multi-vibrator-driven transistor TR7. However, the positive side ofthe front lobby use lamp 9 is connected to the positive bus by diode D22and the untransferred contact CR3-A of relay CR3 associated with thefront lobby which is not energized since a front lobby call switch 20-1,20-n has not been activated. Hence, the intermittent grounding of thelow voltage side of front lobby use lamp 9' by transistor TR7 iseffective to flash the front lobby use lamp at the frequency of themulti-vibrator 106, indicating to prospective users of the front lobbycall panel that the system is in use.

The energization of relay CR2 and the transfer of contact CR2-D inresponse to actuation of a rear lobby call switch 20-1 also connects thepositive bus via diode D10 to a timing circuit 110 which includes aunijunction transistor TR3, a capacitor C2, and charging resistors R40and R17. The connection of the positive bus to the timing circuit causesthe capacitor C2 -to intimate charging. When the capacitor C2 charges toa potential sufficient to fire the unijunction transistor TR3, thecapacitor C2 discharges through the fired unijunction transistor TR3 anda resistor R19 producing a negative pulse on line 111. The negativepulse on line 111, in a manner to be described, functions to resetcertain portions of the control circuitry if there is no answer from thecalled station 10-1 within a predetermined time interval determined bythe time constant of the timer 1 10.

Energization of relay CR2 associated with the rear lobby in response toactuation of rear lobby call switch 20-1 also functions to transfercontacts CR2-A and CR2-B which constitute the selector switch 8,connecting the rear lobby speaker to the amplifier 22 via the reversalswitch 23. Contact CR2-C of energized relay CR2 also transfers to applythe amplified audio frequency tone output on line 102 to common signalline S and in turn via activated call panel switch 20-] to line 81.Since line S1 is unique to remote unit 10-1 associated with activatedcall switch 20-1, the amplified audio frequency tone is transmitted toremote unit 10-1 and only to this remote unit, causing the speaker 26 atthis remote unit to produce an audible tone summoning the called party.

The amplified tone present on line 102 is also applied via capacitor C7to the input of central unit amplifier unit 22 which after amplificationapplies the tone signal to the rear lobby speaker 17 via the reversalswitch 23 which is normally connected such that the output of the lobbyamplifier 22 is input to the selector switch 8,-

which in turn is input to the selected lobby speaker, in this case rearspeaker 17. Thus, in response to activation of rear lobby call switch20-1, an audible tone is transmitted to the speaker of called remoteunit 10-1, as well as to the rear lobby speaker 17. This tonetransmission to the called unit and the lobby whereat the calling partyis located continues as long as the call switch 20-1 is activated.

When the call switch 20-1 is de-actuated, or released, the path isinterrupted from the positive bus to ground via resistor R28 which isshort-circuited by conducting transistor TR8, resistor R46, line C,speaker 26 of remote unit 10-1, line S1, line S, and resistors R38 andR31. Interruption of this circuit turns transistor TR16 off, which inturn turns off transistors TR8 and TR9. Transistor TR9 turns off thetone generator 24, while transistor TR8 removes the shortcircuit fromresistor R28. Interruption of the above described path by release ofcall switch 20-1 also turns off transistor TR12. However, transistorTR13 remains conducting by virtue of the application to its base circuitof a positive bias from the positive bus provided by transferred contactCR2-D of energized relay CR2, which relay was energized resulting in theconduction of transistor TR13 and the latching of relay CR2 whentransistor TR12 was initially energized by activation of call switch20-1.

If there is no answer from the called remote unit 10-1, that is, ifneither the talk nor the listen switches 30 or 32 is activated, thetimer circuit 110 eventually produces a negative reset signal on line111 to switch transistor TR13 off, in turn de-energizing relay CR2 whichdeactivates the flashing multi-vibrator 106 causing flashing front uselamp 9' to extinguish, and disconnects the rear speaker 17 via selectorswitch 8 from the amplifier 22. Specifically, the negative signalproduced on line 111 from the timer circuit 110 in response todischarging of capacitor C2 via fired unijunction transistor TR3 appliesa negative bias to the base of conducting transistor TR13 via capacitorC5, causing this transistor to be rendered nonconductive. Withtransistor TR13 off, relay CR2 is de-energized, transistor TR12 thealternative energization path for relay CR2 having been renderednonconductive when the call switch 20-1 was released. De-energization ofrelay CR2 transfers its contacts CR2-A and CR2-B of selector switch 8 tothe position shown, disconnecting the rear speaker 17 from the amplifier22. Additionally, relay contact CR2-D transfers to the position shown,removing a connection from the positive bus to the flashingmulti-vibrator 106 causing this circuit to cease switching of transistorTR7 and in turn the intermittent grounding of the ground sides of frontand rear use lamps 9' and 9. Front use lamp 9', the lamp which whenrelay CR2 was energized had its positive side connected to the positivebus via diode D22 and non-transferred contact CR3-A associated withfront lobby relay CR3, ceases to be operated in the flashing mode as aconsequence of the termination of the flashing operation ofmulti-vibrator 106. Transfer of contact CR2-D to the position shown inresponse to reset of relay CR2 by the timer 110 also removes theconnection from the positive bus to the timer circuit 110 provided bydiodes D10 and resistors R40 and R17. Hence, timer capacitor C2 does notbegin charging to initiate a new timing cycle. Finally, de-energizationof relay CR2 transfers contact CR2C to the position shown, interruptingthe circuit between common signal line S and the amplified output of thetone generator on line. 102.

As noted previously, a person in the rear lobby 16 desiring tocommunicate with remote unit 10-1 initiates a communication by actuatingcall panel switch 20-1. This, in a manner described in detail earlier,transmits a tone to the speaker of called remote unit 10-1 so long asthe call switch 20-1 is activated. Additionally, activation of the callswitch 20-1 causes relay CR2 associated with the rear lobby to latch.With rear lobby control relay CR2 latched, flasher circuit 106 isenergized via transferred contact CR2-D and front lobby use lamp 9operated in an intermittent flashing mode via untra'nsferred contactCR3-A associated with front lobby control relay CR3. Additionally, timercircuit 110 is energized via transferred contact CR2-D and the rearlobby speaker 17 is connected via transferred contacts CR2-A and CR2-Bto the reversal switch 23. The reversal switch 23, as will be describedlater, is in its normal condition in which rear speaker 17 is connectedto the output of the amplifier 22 and audio lines A1 and A2 areconnected to the input of the amplifier 22. Assuming the call switch20-1 has been released, transistors TR12 and TR16 are nonconductive asare transistors TR8 and TR9, and tone generator transistor TRZ. RelayCR2, however, is latched ON via a conducting transistor TR 13, which isbiased to a conduction state by transferred contact CR2-D of latchedrelay CR2.

With the system in the condition outlined above, if the personnel atcalled remote unit 10-1 desire to speak to the calling party in the rearlobby, the called party activates the talk switch 30, moving it from theposition shown in solid lines to the position shown in phantom lines.Activation of the talk switch 30 moves the speaker 26 of the called unit10-1 from across control line C and signal line S1 and places thespeaker across audio lines A1 and A2. Since the normal condition ofreversal switch 23 is such that audio lines A1 and A2 are connected tothe input of amplifier 22, and the speaker, in this case rear lobbyspeaker 17, selected by selector switch 8 is connected to the output ofamplifier 22, the called party upon actuation of the talk switch 30 isnow in a position to communicate with the calling party in the rearlobby 16. The voice communication from the called party will be audiblyreproduced by the rear lobby speaker 17. The voice communication willnot, however, be reproduced by speaker 17 in the front lobby 16 sincefront lobby speaker 17' is not connected to the output of the amplifierby the selector switch 8.

Further, the voice communication from the called unit 10-1 cannot bereceived by any other remote units. Specifically if another remote unit10-2, 10-n were to have its talk switch 30 activated, the speaker ofthat remote unit, like the remote speaker of called remote unit 10-1,would not be connected via audio lines A1 and A2 to the output ofamplifier 22, and hence could not receive the voice communication fromcalled unit 10-1. If a remote unit other than called unit 10-1 were tohave its listen switch activated, it could not receive the communicationfrom called unit 10-1, for reasons to become apparent later, becauseactivation of the listen switch of the uncalled unit would be effectiveto switch the condition of the reversal switch 23 such that the speaker26 of called unit 10-1 is not connected via audio lines Al and A2 to theinput of amplifier 22. Therefore, even though activation of listenswitch 32 at an uncalled remote unit is effective to switch the speakerof the uncalled remote unit to the output of amplifier 22, there is novoice communication being amplified by amplifier 22 to be received bythe uncalled unit since speaker 26 of called unit 10-1 is no longerconnected to the input of amplifier22 via audio lines A1 and A2.

If after talking to the rear lobby unit 16 the personnel at called unit10-] desires to terminate the communication, the talk switch 30 thereatis released. This removes speaker 26 of called unit 10-1 from acrossaudio lines A1 and A2 and returns the called unit to the normal orstandby position with the speaker across signal line S1 and control lineC. Rear lobby speaker 17 remains connected via transferred contactsCR2-A and CR2-B of selector switch 8 to the reversal switch 23 and henceto the output of the amplifier 22 until relay CR2 is de-energized by areset signal on line 111 from the timing circuit in a manner describedpreviously.

If after talking to the rear lobby unit 16, the personnel at called unit10-1 desires to permit the calling party to speak, the called partyactivates the listen switch 32, placing the system in the listen mode.Of course, the listen switch 32 must be activated before the timercircuit 110 has reset the system, particularly de-energized relay CR2 byproduction of a negative reset signal on line 11 1. Assuming that thelisten switch 32 at called unit -1 has been activated before the timer110 has had an opportunity to produce the reset signal on line 111 tode-energize relay CR2, activation of the listen switch 32 willeffectively place the system in the listen mode. Specifically,activation of the listen switch 32 at called unit 10-1 places thespeaker 26 of that remote unit across audio lines Al and A2.

Additionally, activation of the listen switch places resistor R48 acrosscontrol line C and audio line A2. Placement of resistor R48 acrosscontrol line C and audio line A2 causes current to flow from thepositive bus to ground via resistor R28, resistor R46, control line C,resistor R48, audio line A2, and the primary winding W1 of transformerT1. The effect of current flowing in this path causes the voltage ofcontrol line C to decrease which in turn places an increased voltageacross zener diode D11 connected in the base circuit of a transistorTR15. The increased voltage drop across zener diode D11 is sufficient tocause this diode to break down, switching normally nonconductingtransistor TRIS to a conductive state. Transistor TRIS has itsemitter-collector path connected in series with a relay CR1 and whenswitched to a conductive state completes an energization path for relayCR1 through line 120, diode D4, line 121, and the emitter-collector pathof conducting transistor TR13 which is maintained in a conductive stateby transferred contact CR2-D of control relay CR2, which it will berecalled was energized when call switch 20-1 was activated to callremote unit 10-1. Relay CR1 remains energized only so long as the listenswitch 32 is activated.

Energization of relay CR1 in response to activation of the listen switch32 at called unit 10-1 is effective to transfer contacts CR1-A and CR1-Bforming part of the reversal switch 23 to connect the rear lobby speakervia selector switch 8 to the input of amplifier 22. Additionally,contacts CR1-C and CR1-D, also forming part of the reversal switch 23,transfer from the position shown to connect audio lines A1 and A2 and,hence, remote speaker 26 of the remote unit 10-l to the output ofamplifier 22. With rear lobby speaker17 connected to the input ofamplifier 22 and the speaker 26 of remote unit 10-1 connected to theoutput of amplifier 22, the system is in the listen mode, permittingconversation to be transmitted from the rear lobby 16 to called unit10-1. As noted, the system will remain in the listen mode so long as thelisten switch is held in the active position shown in phantom lines.

Energization of relay CR1 in response to activation of the listenswitch, in addition to reversing the condition of reversal switch 23 toconnect the lobby and remote unit speakers to the input and output,respectively, of amplifier 22, also performs certain additionalfunctions. Specifically, relay contact CR1-E transfers from the positionshown to discharge timing capacitors C2 effectively resetting the timer.Since contact CR l-E remains transferred from the position shown so longas relay CR1 is energized by activation of the listen switch, thecapacitor C2 remains short-circuited for the duration of the listenmode, preventing the timer 110 from beginning a timing cycle.

Relay contact CR1-F also transfers from the position shown to ground thelow voltage side of relay CR1 via line and diodes D4 and D6. Thus,continued eneri gization of relay CR1 is no longer dependent uponcontinued conductivity of transistor TR13. Transferred contact CR1-F isalso effective to ground via diode D6 control relay CR2 such thatcontinued conductivity of latched relay CR2 is no longer dependent uponcontinued conduction of TR12. It is desirable that continued conductionof relay CR1 and CR2 not be dependent upon continued conduction oftransistor TR13 since there is a tendency for this transistor to berendered nonconductive when contact CR1-E transfers from the positionshown and discharges timing capacitor C2. Specifically, when contactCR1-E transfers from the position shown and discharges timing capacitorC2, a negative signal is produced on timer output line 1 11 which has atendency to terminate conduction of transistor TR13. Thus, relays CR1and CR2 which, prior to transfer of contact CR1-F, depended forcontinued conductivity on the conduction of transistor TR13, would ceaseconducing were it not for transferred contact CR1-F which in effectshort-circuits the low voltage side of relay CR1 and CR2 to groundindependent of transistor TR 13.

When the called party at remote unit 10-1 desires to terminate thelisten mode, that is, end conversation originating at the central unit,the listen switch 32 is deactivated and returned to the solid lineposition. This is effective to remove speaker 26 of remote unit 10-1from across audio lines A1 and A2 and transfer the speaker to lines S1and control line C. Additionally, resistor R48 is no longer connectedbetween control line C and audio line A2, terminating the path from thepositive bus to ground via resistors R28, R46, control line C, resistorR48, and primary winding S1 of transformer T1. With this conductor pathterminated, the voltage across zener diode D11 decreases to a pointwherein the diode is no longer in a breakdown condition and transistorTRIS is rendered nonconductive, in turn interrupting the energizationpath for relay CR1 causing this relay to become de-energized. Relaycontacts CR1-A and CR1-B transfer to the position shown, disconnectinglobby speaker 17 from the input of amplifier 22 and connecting it to theoutput of amplifier 22. Contacts CR1-C and CR1-D return to the positionshown transferring audio lines Al and A2 from the output of amplifier 22to the input of amplifier 22. Contact CR1-E transfers to the positionshown, removing the short-circuit from timing capacitor C2. Withcapacitor C2 no longer short-circuited, it is free to charge throughtransferred contact CR2-D, diode D10, and resistors R40 and R17. ContactCR1-F also transfers to the position shown. Relay CR2 which is nowenergized is connected to the ground bus via conducting transistor TR13rather than via contact CR1-F, diode D6 and line 121. Thus, control ofde-energization of conducting relay CR2 is now returned to transistorTR13.

When timing capacitor C2 has charged to a value sufficient to fireunijunction transistor TR3, a negative reset signal is produced onoutput line 111 which is capacitively coupled via capacitor C5 to thebase of conducting transistor TR13, rendering this transistornonconductive. Nonconduction of transistor TR13 interrupts'theenergization circuit for relay CR2 causing this relay to becomede-energized. De-energization of relay CR2 in the manner describedpreviously disconnects the rear speaker 17 from the reversal switch 23and hence from the amplifier 22; terminates operation of the flashermulti-vibrator 106 and hence extinguishes the front use lamp 9;terminates charging of the capacitor C2 via diode D10 and resistors R40and R17; and disconnects the output of amplifier transistor TR17 fromcommon signal line S.

Of course, prior to the production of the reset signal on line 111 fromtimer circuit 110 personnel at the called remote unit 10-1 can place thesystem in the talk mode by activating switch 30 to permit voicecommunication from the called unit to the rear lobby 16 in a mannerdescribed. Activation of the talk switch could then be followed byactivation of the listen switch to place the system in the listen modewhich, as noted, would have the effect of resetting the timer 110 andholding it in an inoperative condition until the listen switch is againde-activated.

If desired, and before relay CR2 is de-energized by a reset signal online 111 from timer circuit 110, personnel at the called unit 10-1 mayactivate the rear door release mechanism 18. Specifically, the rear doorrelease mechanism 18 is activated by actuating door release switch 28.This is effective to place resistor R49 between control line C and audioline A2 which completes a circuit from the positive bus through resistorR34, zener diode D1, resistor R36, control line C, resistor R49, audioline A2, and primary winding W1 of transformer T1. Completion of thiscircuit decreases the voltage of control line C, placing an increasedvoltage across zener diode D1, which is sufficient to break down thisdiode. Breakdown of zener diode D1 biases normally nonconductingtransistor TR14 to a conductive state. Transistor TR 14 which has itsemitter-collector path in series with a control relay CR4, when renderedconductive, causes relay CR4 to be energized via a path which includesdiode D4, line 121, and the emitter-collector path of conductingtransistor TR13. It will be recalled that transistor TR13 is biased to aconductive state when contact CR2-D transfers to the position shown uponenergization of relay CR2 when call switch 20-1 is actuated.

Contact CR4B transfers from the position shown to complete anenergization circuit from a power supply 125 to the rear door releasemechanism 18 viauntransferred contact CR3D of control relay CR3associated with the front lobby. Control relay CR3 is the front lobbycounterpart of control relay CR2 and remains unenergized so long as acall switch 20-1 20-n in the front lobby is not activated. Thus, as longas the calling party is located in the rear lobby, relay contact CR3-Dremains in the position shown to partially complete an energizationcircuit from the power supply 125 to the rear door release mechanism 18.When the door release switch 28 in the remote unit is activated toenergize relay CR4, the partially completed circuit to the rear doorrelease mechanism 18 via untransferred contact CR3-D is completed by thetransfer of contact CR4B. Had the calling party been located in thefront lobby, relay CR3 would be energized and contact CR3-D transferredfrom the position shown, such that when contact CR4-B transfers inresponse to energization of relay CR4 as a consequence of activating thedoor release switch 28, the front door release mechanism 18' would beactivated instead of the rear door release mechanism 18.

Energization of relay CR4 also transfers contact CR4-A which completes acircuit from a positive bus to the relay CR4 to latch this relayindependent of transistor TR14, the energization state of which iscontrolled by door release switch 28. Transfer of contact CR4- A alsocompletes a second charging path for capacitor C2 via diode D12,resistor R18 and resistor R41. This second charging path is in additionto the path through diode D10, resistor R40 and resistor R17 establishedby transferred contact CR2-D. With two charge paths established fortimer capacitor C2, the capacitor charges at an increased rate untileventually unijunction transistor TR3 fires and capacitor C2 dischargesto produce a negative reset signal on line 111 output from timer circuit110. The reset signalon line 111 is capacitively coupled via capacitorC5 to the base circuit of conducting transistor TR13, causing thistransistor to switch off, in turn de-energizing relay CR2.De-energization of relay CR2 disconnects the rear lobby speaker 17 fromthe reversal switch 23 and, hence, the amplifier 22; interrupts thecharging circuit for capacitor C2 via diodes D10 and resistors R40 andR17; interrupts the energization circuit for the flasher multi-vibrator106, preventing flashing of the front use lamp 9'; and disconnectsoutput line 102 of amplifier TR17 from common signal line S. Turning offof transistor TR13 in response to the reset signal on line 111 fromtimer circuit also interrupts the path to ground bus for relay CR4,causing relay CR4 to become tie-energized. De-energization of relay CR4transfers contact CR4-B to the position shown, interrupting theenergization circuit from source to the rear door release mechanism 18.Contact CR4-A also returns to the position shown interrupting the chargepath for capacitor C2 via diodes D12 and resistors R18 and R41, as wellas removing the latch circuit from the positive bus for relay CR4. Theentire system is now reset.

If desired, the rear door unlocking mechanism 18 can be energizedindependently of the door release switches 28 located inremote units10-1, 10-2, l0-n. For example, a manually operated switch may beprovided which when activated, for example, with a key, completes acircuit from the positive bus to the ground bus via resistor R34,resistor R35, line 141, resistor R5, and resistor R6. Completion of thiscircuit switches normally nonconducting transistor TR12 to a conductivestate, which in turn energizes relay CR2 through normallynon-transferred contact CR3-A, resistor R45 and diode D15. Energizationof relay CR2 transfers contact CR2-D to switch normally nonconductingtransistor TR13 on. With transistor TR13 conducting, a latch circuit isestablished for relay CR2 independent of transistor TR12, the conductivestate of which is responsive to, among other things, keyoperated switch140. Transfer of contact CR2-D also establishes a charging path throughdiode D10, resistor R40 and resistor R17 for timing capacitor C2 oftiming circuit 110. Actuation ofkey-operated switch 140 also biasesnormally nonconducting transistor TR14 to a conductive state, which inturn energizes relay CR4 via diode D4 and line 121 and conductingtransistor TR13. Energization of relay CR4 transfers contact CR4-B tocomplete an energization circuit from the power supply 125 to the reardoor release mechanism 18 via normally untransferred contact CR3-D.Additionally, relay contact CR4-A transfers to latch relay CR4 in aconductive state independent of transistor TR14, the conductive state ofwhich is controlled by switch 140. Transfer of contact CR4-A alsocompletes a second charging circuit for capacitor C2 through diode D12,resistor R18 and resistor R41. When timing capacitor C2 charges to apoint sufficient to fire unijunction transistor TR3, a negative resetsignal is produced on line 111 of timing circuit 110 which is coupledvia capacitor C5 to the base of conducting transistor TR13, renderingthis transistor nonconductive and in turn de-energizing relay CR2 andrelay CR4, which rely on conduction of transistor TR13 for completion ofa circuit to ground. De-energization of relay CR4 transfers contactCR4-B to the position shown de-energizing the rear door releasemechanism 18. The entire circuit is now reset.

The foregoing description of the operation of the system assumes that acall switch in the rear lobby 16 was activated. Operation of the systemif a call switch in the front lobby 16' is activated is the same as ifthe call switch in the rear lobby were activated with the followingexceptions. Transistor TR switches ON instead of transistor TR12, withthe result that relay CR3 is energized and not relay CR2. Additionally,transistor TR] 1 becomes energized to latch relay CR3 in an energizedcondition in much the same manner that transistor TR13 latched relay CR2when a rear lobby call panel switch was activated; Energization of relayCR3 in response to activation of a front panel call switch transferscontact CR3-C from the position shown. Transfer of contact CR3-Cconnects the positive bus to the flasher multi-vibrator 106 which inturn intermittently via transistor TR7 connects the ground bus to theuse lamps 9 and 9. However, the transfer of contacts CR3-A interruptsthe connection from the positive bus through diode D22 to the front uselamp 9' preventing this lamp from being flashed. Rear use lamp 9 isflashed by the intermittent grounding because its positive side isconnected to the positive bus via diode D17 and untransferred contactCR2-D of relay CR2 which, as indicated, is not energized when a frontpanel call switch is activated. Transfer of contact CR3-C also connectsthe positive bus to the base of transistor TR] 1 to, as indicatedpreviously, latch transistor TRll in a conductive state, in turnlatching relay CR3 in an energized condition.

Transfer of contact CR3-A completes a charging circuit from the positivebus through charging resistors R40 and R17 to the timing capacitor C2 oftimer circuit 110. This capacitor charging circuit functions in a mannersimilar to that described in connection with the previous discussionwherein relay CR2 is energized in response to actuation of a rear lobbycall switch. Relay contact CR3-D transfers from the position shown topartially complete a circuit from the power supply 125 to the front doorrelease mechanism 18'. With this partial circuit completed, when thedoor release switch 28 is actuated and relay CR4 energized, contactCR4-B transfers to complete an energization circuit to the front doorrelease mechanism 18.

The front door speaker 17 is connected to the reversal switch 23 and,hence, to the amplifier 22 via nontransferred contacts CR2-A and CR2-B.Hence, the system is in condition for communication between the remotestation and the speaker of the front lobby.

Resetting of the system in response to a reset signal on line 111 fromthe timer circuit when a front lobby call switch is activated is similarto that previously described in connection with activation of a rearpanel call switch. However, the reset signal on line 1 l 1 is coupledvia a capacitor C6 to the base circuit of transistor TRll turning thistransistor off which in turn de-energizes relay CR3. De-energization ofrelay CR3 transfers contact CR3-C to the position shown, deenergizingthe flashing circuit 106, enabling front lobby use lamp 9, andinterrupting the charging path to the timing capacitor C2 via resistorsR40 and R17.

Operation of the front door release mechanism 18 in response toactivation of the door release switch 28 is similar when a front lobbycall panel switch is operated to the operation when a rear call panelswitch is operated. However, relay CR4 completes a circuit to ground viadiode D3 and conducting transistor TR11 rather than diode D4, line 121and conducting transistor TR13. This ground circuit for conducting relayCR4 is interrupted when transistor TRll is rendered nonconductive inresponse to a reset signal on line 111 coupled to the base of transistorTRll by capacitor C6. When the front lobby is being used and the listenswitch of a remote unit is activated, relay CR1 completes a circuit toground through transistor TRll rather than transistor TRIS. Finally,when the front lobby is being used, the amplified tone on line 102 iscoupled to common signal line S via transferred contact CR3-B instead ofcontact CR2-C.

To reset the timer circuit 1 10 each time a call switch 20-1, 20-n or20-1, 20-n' is activated, a transistor TR4 is provided having itsemitter-collector path in parallel with timing capacitor C2 and its baseconnected to the output of the tone generator amplifier TRl7 on line102. When a call switch is activated and an amplified tone present online 102, transistor TR4 is rendered conductive causing capacitor C2 todischarge effectively resetting the timer 110. Thus, personnel at theremote units are permitted the full duration of the timer 110 to respondto a call.

Diode D16 connected between the collector of transistor TR14 and relaycontact CR4-A prevents switching of transistor TR10 to a conductivestate and in turn energization of relay CR3 to activate the rear doorrelease 18 via contact CR4-B if the door release switch 28 of a remoteunit 10-1, 10-n is activated when neither the front nor rear lobby is inuse. Without diode D16, closure of a door release switch 28 in a remoteunit 10-1, l0-n would switch transistor TR14 on, in turn switchingtransistor TR10 on to energize relay CR3 and complete a circuit to therear door release mechanism 18 via tr'ansferredcontact CR3-D andtransferred contact CR4-B. Contact CR4-B would have transferred uponenergization of relay CR4 as a consequence of conduction of transistorTR14 when the door release switch 28 was activated.

Having described my invention, I claim:

1. An intercommunication system comprising n remote stations each havinga two-way speaker,

a central station having a first two-way speaker,

first and second audio lines each connected to each of said n remotestations and connectable to said central station two-way speaker,

audio frequency signal generator means,

it signal lines each connected between said signal generator means and adifferent one of said n remote stations,

a first set of n call switches at said central station and associatedwith said first speaker, each of said switches corresponding to adifferent one of said n remote stations for transmitting an audiofrequency signal between said signal generator means and a selectedremote station via the signal line connected to the selected remotestation,

a single common control line connected to each of said n remote stationsand said central station,

audio frequency amplifier means connected in said audio lines,

amplifier control means connected to said amplifier and between saidsingle control line and at least one of said audio lines, said controlmeans responsive to a control signal on said control and at least oneaudio line from said remote stations for amplifying, on an alternativebasis, communications on said audio lines in one direction from saidremote stations and in another direction from said central station,

n switch means respectively at different ones of said remote stations,each of said switches normally connecting its respective two-way speakerbetween its respective signal line and said common control line tofacilitate, without manipulation of its remote station switch means,audible reproduction of an audio frequency signal transmitted from saidsignal generator means to said remote station over said respectivesignal line and said common control line in response to activation fromsaid central station of the call switch corresponding to said remotestation, said 11 switch means each having a second condition in whichits respective twoway speaker is connected between said audio lines, andsaid n switches each having a third condition in which its respectivetwo-way speaker is connected between said audio lines and in which acontrol signal is placed on said control and at least one audio line tocause said amplifier control means to alter the direction ofamplification of said amplifier.

2. The system of claim 1 wherein central station has a second two-wayspeaker remote from said first two-way speaker; and further including acentral station speaker selector switch connected between said first andsecond central station speakers and said amplifier means foralternatively connecting said central station speakers to said amplifiermeans; a second set 11 call switches at said central station andassociated with said second speaker, each of said n switchescorresponding to a different one of said n remote stations fortransmitting an audio frequency signal between said signal generatormeans and a selected remote station via the signal line connected to theselected remote station; and call switch responsive control meansresponsive to said first and second sets of call switches forcontrolling said selector switch to connect to said amplifier means onlyone of the central station speakers when a call switch is actuated, theone central station speaker connected being that WhlCh 15 associatedwith the call switch set having the actuated call switch.

3. The system of claim 2 wherein said remote station switch means eachhave a fourth condition wherein a door unlock signal is placed acrosssaid control line and one of said audio lines; said system furtherincluding first and second door control mechanisms for controlling doorsassociated with said first and second central station speakers,respectively, said door control mechanisms being jointly responsive tosaid call switch responsive control means and to said door unlocksignals for actuating, in response to a door unlock signal, only thedoor unlock mechanism associated with the call switch set having theactuated call switch.

1. An intercommunication system comprising n remote stations each havinga two-way speaker, a central station having a first two-way speaker,first and second audio lines each connected to each of said n remotestations and connectable to said central station two-way speaker, audiofrequency signal generator means, n signal lines each connected betweensaid signal generator means and a different one of said n remotestations, a first set of n call switches at said central station andassociated with said first speaker, each of said switches correspondingto a different one of said n remote stations for transmitting an audiofrequency signal between said signal generator means and a selectedremote station via the signal line connected to the selected remotestation, a single common control line connected to each of said n remotestations and said central station, audio frequency amplifier meansconnected in said audio lines, amplifier control means connected to saidamplifier and between said single control line and at least one of saidaudio lines, said control means responsive to a control signal on saidcontrol and at least one audio line from said remote stations foramplifying, on an alternative basis, communications on said audio linesin one direction from said remote stations and in another direction fromsaid central station, n switch means respectively at different ones ofsaid remote stations, each of said switches normally connecting itsrespective two-way speaker between its respective signal line and saidcommon control line to facilitate, without manipulation of its remotestation switch means, audible reproduction of an audio frequency signaltransmitted from said signal generator means to said remote station oversaid respective signal line and said common control line in response toactivation from said central station of the call switch corresponding tosaid remote station, said n switch means each having a second conditionin which its respective two-way speaker is connected between said audiolines, and said n switches each having a third condition in which itsrespective two-way speaker is connected between said audio lines and inwhich a control signal is placed on said control and at least one audioline to cause said amplifier control means to alter the direction ofamplification of said amplifier.
 2. The system of claim 1 whereincentral station has a second two-way speaker remote from said firsttwo-way speaker; and further including a central station speakerselector switch connected between said first and second central stationspeakers and said amplifier means for alternatively connecting saidcentral station speakers to said amplifier means; a second set n callswitches at said central station and associated with said secondspeaker, each of said n switches corresponding to a different one ofsaid n remote stations for transmitting an audio frequency signalbetween said signal generator means and a selected remote station viathe signal line connected to the selected remote station; and callswitch responsive control means responsive to said first and second setsof call switches for controlling said selector switch to connect to saidamplifier means only one of the central station speakers when a callswitch is actuated, the one central station speaker connected being thatwhich is associated with the call switch set having the actuated callswitch.
 3. The system of claim 2 wherein said remote station switchmeans each have a fourth condition wherein a door unlock signal isplaced across said control line and one of said audio lines; said systemfurther including first and second door control mechanisms forcontrolling doors associated with said first and second central stationspeakers, respectively, said door control mechanisms being jointlyresponsive to said call switch responsive control means and to said doorunlock signals for actuating, in response to a door unlock signal, onlythe door unlock mechanism associated with the call switch set having theactuated call sWitch.